1. Field of the Invention
The present invention relates to a belt member having a seamed portion and to an image forming apparatus having this belt member. This image forming apparatus is suitable for use as a color-type image forming apparatus in which images of a plurality of colors are superimposed one upon the other on a recording material carried by the belt member.
2. Description of the Related Art
Conventionally, there have been proposed various types of color image forming apparatuses. In these image forming apparatuses, a plurality of image forming sections are provided, and toner images of different colors are respectively formed in these image forming sections, the toner images being transferred to the same recording material and superimposed one upon the other. Of such image forming apparatuses, a color copying apparatus of a multicolor electrophotographic type is in wide use.
An example of such a color electrophotographic copying apparatus will be briefly described with reference to FIG. 6. In the body of the color electrophotographic copying apparatus, first through fourth image forming sections Pa, Pb, Pc and Pd are arranged side by side. The image forming sections Pa through Pgd are equipped with electrophotographic photosensitive drums 1a, 1b, 1c and 1d, respectively, which serve as dedicated image carrying means.
Around the photosensitive drums 1a through 1d, there are arranged latent image forming sections 2a, 2b, 2c and 2d, developing sections 3a, 3b, 3c and 3d, and cleaning sections 5a, 5b, 5c and 5d, respectively.
Under the image forming sections Pa through Pd, a conveyor belt 8 for conveying the recording material to the respective image transfer positions is arranged such that it can run by means of a driving roller 12 and a driven roller 11. Inside the conveyor belt 8, transfer discharge sections 4a, 4b, 4c and 4d are arranged respectively in correspondence with the photosensitive drums.
In this construction, first, a latent image of cyan color component is formed on the photosensitive drum 1a of the first image forming section Pa by the latent image forming section 2a on the basis of image information read from an image reader (not shown). This latent image is turned into a visual image by using a developer of the developing section 3a having cyan toner. Then, the image transfer section 4a transfers the cyan toner image onto a recording material 6, which is supplied from a recording material cassette 60, fed by way of registration rollers 13 and conveyed by the conveyor belt 8.
While the cyan image is being transferred to the recording material 6 as described above, a latent image of magenta color component is formed by the second image forming section Pb. Then, this latent image is turned into a magenta toner image by the developing section 3b and transferred onto a predetermined position of the recording material 6 so as to be superimposed on the cyan toner image when the recording material 6, which has undergone image transfer in the first image forming section Pa, is fed to the image transfer section 4b.
By similar processes, yellow and black images are formed by the third and fourth image forming sections Pc and Pd, and the yellow and black colors are transferred onto predetermined positions of the same recording material 6 so as to be superimposed on the cyan and magenta images.
When these image forming processes have been completed, the image on the recording material 6 is fixed to the recording material 6 by a fixing section 7, whereby a multicolor image is completed.
After the completion of the image transfer, the toner remaining on the photosensitive drums 1a through 1d is removed by cleaning sections 5a through 5d, and the apparatus made ready for forming the next latent image.
The conveyor belt 8, which is an endless belt member, consists of a dielectric resin film, such as a polyethylene terephthalate resin sheet (PET sheet), a polyvinylidene fluoride resin film, or a polyurethane resin film. The end portions of the belt are superimposed one upon the other to be joined together to be thereby formed into an endless belt. Alternatively, a so-called seamless belt having no seam may be used.
However, of the above-mentioned conventional belts, the seamless belt is rather difficult to produce, and has serious problems in terms of productivity, cost, etc.
On the other hand, the so-called seamed belt having a seam involves a problem when image transfer is effected on that portion of the recording material which is being held by the belt portion corresponding to the seamed portion; the portion of the recording material superimposed on the seam portioned has physical properties that are different from those of the remaining portion of the recording material, so that the transfer electric field of this portion of the recording material differs from that of the remaining portion thereof, resulting in an image disturbance. As a result, a defective image is generated, which has inconsistency in density appearing in the form of a line corresponding to the seamed portion.
To prevent the generation of such inconsistency in density, various measures have been proposed. For example, use of the belt is started from a position which is spaced apart from the seam position by a predetermined distance in order that the recording material may not be placed on the seamed portion.
Further, a method has been proposed according to which, in order that the recording material may not be placed on the seamed portion, the seamed portion is detected to adjust the timing with which the recording material is placed on the conveyor belt.
However, the above-mentioned seamed belt still has the following problems:
The first problem relates to the strength of the belt. The belt is stretched between a plurality of conveying rollers and conveys the recording material as it runs, so that it is repeatedly wound around the conveying rollers and separated therefrom. That is, the belt repeatedly receives bending stress, which means the fatigue strength of the belt is an important factor to be taken into consideration.
The bonding of the seamed belt is effected by adhesion bonding using an adhesive, heat bonding using heat, etc. In any case, the fatigue strength of the bonded section cannot exceed that of the remaining portion of the belt, with the result that cracks are generated in the bonded section after being used for a long period of time, which leads to a break in the belt. Thus, the service life of the belt is rather short.
Japanese Unexamined Patent Publication No. 63-280277, for example, discloses a structure in which, in order to prevent erroneous detection by a sensor, the seam of the transfer belt extends in a direction crossing the direction perpendicular to the movement of the belt. This structure, however, has a problem in terms of the belt strength.
Regarding the above problem, it is known that a substantial improvement can be achieved by attaching a thin adhesive tape (consisting, for example, of a polyester or a teflon (trade name) tape), which constitutes a reinforcing member, to the surface of the bonded section of the belt. However, even this arrangement is insufficient in preventing the generation of cracks starting from the end portions with respect the width direction of the belt. Thus, a further improvement is required in this regard.
Further, both the conveyor belt as the conveying means and the photosensitive belt as the image carrying means have cleaning means which are in contact with their surfaces, and when the step portion formed by the adhesive tape passes the cleaning means, vibrations and uneven rotation, which lead to displacement of the belt, are generated, resulting in unevenness in the image. In particular, in the case of a color image forming apparatus, such vibrations and uneven rotation will lead to color misregistration.